class Animal:
    def eat(self):
        raise NotImplementedError("Subclasses must implement this method")

class Dog(Animal):
    def eat(self):
        return "吃骨头"

class Cat(Animal):
    def eat(self):
        return "吃鱼"

class Cow(Animal):
    def eat(self):
        return "吃草"

def animal_eat(animal):
    print(animal.eat())

dog = Dog()
cat = Cat()
cow = Cow()

animal_eat(dog)
animal_eat(cat)
animal_eat(cow)